"Despite the persistence of erroneous theories about atmospheric chemical spraying programmes, until now there were no peer-reviewed academic studies showing that what some people think are 'chemtrails' are just ordinary contrails, which are becoming more abundant as air travel expands," said Ken Caldeira of the Carnegie Institution for Science, US. "Also, it is possible that climate change is causing contrails to persist for longer periods than they used to. I felt it was important to definitively show what real experts in contrails and aerosols think. We might not convince die-hard believers that their beloved secret spraying programme is just a paranoid fantasy, but hopefully their friends will accept the facts."

In a survey conducted in 2011, nearly 17% of the 3015 people questioned said it was true or partly true that there is a secret government programme that uses aeroplanes to put harmful chemicals into the air. (Some 2.6% said it was "completely true" whilst 14% said it was "partly true".) Social scientists have linked such beliefs to growing public distrust of elites and social institutions. Researchers studying climate geoengineering with computer models, meanwhile, have received threats for their alleged role in a secret spraying programme.

"The chemtrails conspiracy theory maps pretty closely to the origin and growth of the internet, where you can still find a number of websites that promote this particular brand of pseudoscience," said Steven Davis of the University of California, Irvine, US. "Our survey found little agreement in the scientific community with claims that the government, the military, airlines and others are colluding in a widespread, nefarious programme to poison the planet from the skies."

A number of websites claim to show evidence of widespread chemical spraying, also dubbed "chemtrails" or "covert geoengineering", that’s harming human health and the environment. Some people reckon the aim of this perceived spraying is to control population, food supply and/or the weather, whilst others think it’s to geoengineer the climate. The websites typically display data such as photos of aircraft trails and analyses of water, soil and snow samples that are claimed to contain higher-than-normal levels of strontium, barium and aluminium.

For their survey of experts, Caldeira, Davis and colleagues Christine Shearer and Mick West questioned 77 scientists, a mix of atmospheric chemists with expertise in condensation trails and geochemists working on atmospheric deposition of dust and pollution. A total of 76 said they had not encountered evidence of a secret large-scale atmospheric programme.

"We wanted to establish a scientific record on the topic of secret atmospheric spraying programmes for the benefit of those in the public who haven't made up their minds," said Davis. "The experts we surveyed resoundingly rejected contrail photographs and test results as evidence of a large-scale atmospheric conspiracy."

The team showed experts four photos of contrails and three elemental analyses of samples, from pond sludge, airborne particulates and a snow surface.

None of the experts thought that the contrails in the photos were best explained by chemical spraying. For example, they cited the presence of pockets of dry or warm air, or subsidence or upwelling as the cause for a gap in a contrail, rather than a temporary switch-off of chemical spraying.

The bulk (80–89%) of respondents did not think the simplest explanation for each of the three sample analyses involved chemical spraying. Several respondents raised concerns about the sampling technique recommended on one of the chemtrail websites, as it could lead to contamination of the samples by the Mason jars suggested for collecting them.

"The jar will contaminate the sample, as will the metal lid, particularly if you shake it," said one expert. "I cannot imagine a worse protocol for collecting a sample; the data would be totally worthless." According to another scientist, "to analyze metals in environmental samples, glass needs to go through an acid wash to remove any residual metals. Otherwise, plastic should be used."

In 2000, the US Environmental Protection Agency, Federal Aviation Administration, National Aeronautics and Space Administration and National Oceanic and Atmopsheric Administration posted a fact sheet assuring the public that the government is not operating a large-scale atmospheric spraying programme but the claims had not, until now, been addressed in peer-reviewed scientific literature.

"The number of aircraft contrails has been increasing," writes the team in Environmental Research Letters (ERL). "There have been revelations over the decades of governments undertaking action in secret without the informed consent of the population. It is reasonable that ordinary citizens should want questions answered concerning health, climate change and pollution. While we understand that many of the fears underlying SLAP [secret large-scale atmospheric programme] theories may be legitimate, the evidence as evaluated here does not point to a secret atmospheric spraying programme."

The researchers add that changes in aircraft technologies may be causing contrails to persist longer than they used to, and changes in industrial development could potentially be increasing aerosol deposition in some areas. "But the focus on a secret, large-scale atmospheric spraying programme may be taking attention away from real, underlying problems that need addressing," they write.

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